Hybrid-t waveguide structure



April 1-4, 1959 L. LEWIN ET AL HYBRID T WAVEGUIDE STRUCTURE Filed July 26, 1955 Y l l I I II llll Inventors L. L E W IN A. E.P THICK A Home y States Patent HYBRID-T WAVEGUIDE STRUCTURE Application July 26, 1955, Serial No. 524,432

4 Claims. (Cl. 333-11) Eng- Cut-- The invention relates to hybrid waveguide structures of the type frequently referred so as Magic T and is particularly, but not exclusively, concerned with hybrid junction structure suitable for use in channel dropping filter arrangements forming part of multichannel microwave communication systems.

There is already known a hybrid junction which con sists essentially of a wave-guide input arm connected to two like sections of rectangular waveguide which form V-shaped structure with their inner sides forming the inner apex of the structure and their outer sides terminated to form a truncated outer apex having a width substantially equal to the width of either of the sections forming the V, the distance between the outer and inner apices being one half wavelength. A fourth arm is connected into the junction space formed at the said apices by another waveguide which is coupled to the junction space via a coupling probe connected to the fourth arm through a short length of coaxial line which mehcanically supports the fourth arm. In such an arrangement the use of the coaxial line as a support for the fourth arm means that this arm is not very readily connected to the arm of the unit, and this might lead to its taking mechanical strains with consequent deterioration of electrical performance.

It is an object of the invention to provide a junction structure which is mechanically convenient for such channel dropping arrangements and which is well matched over a wide band of frequencies.

In order to achieve these objects there is provided according to the invention a waveguide hybrid junction structure of the Magic T type in which the waveguides comprised in the junction are of rectangular cross-section with sides of ratio substantially equal to three to one, and in which the two (symmetrical) side arms extend parallel to the direction of one of the other arms.

The nature of the invention will be better understood from the following description of an embodiment taken in conjunction with the accompanying drawing, in which Fig. l is a schematic representation of the waveguide paths viewed at a cross section of a structure according to the invention, and Figs. 2, 3 and 4 are different perspective views of an embodiment of the invention. In the drawing the same reference numeral is used to desig nate the same part throughout the several figures.

Referring to the several figures conjointly, there is indicated at 1 an input or E-arm which is centrally con nected into one of the broad walls at the closed end of an H-arm 2 which extends perpendicularly to the E- arm. Two side arms 3 and 4 are connected into the H-arm 2 on the other wall thereof, with their longitudinal axes coplanar with that of arm 1 and extending perpendicularly to the broad wall of the H-arrn. All these arms are of the same rectangular cross-section, the longer dimension of the rectangle being substantially equal to three times the shorter dimension. The side arms 3 and 4 enter the H-arm 2 at separate points such that Patented Apr. .14, 1959 2 the arms 3 and 4 are spaced apart substantially one third of the said longer dimension. The four arms constitute together a unitary structure having a high degree of rigidity.

In multichannel microwave systems it is necessary to secure good impedance matching at all points .in the system, in order to avoid the setting up of interchannel interference. Good impedance matching in hybrid junctions used for channel dropping purposes in such systems is specially important, since there maybe quite a large number of such junctions operating any source. In the present embodiment matching of the E-arm 1 into the junction is obtained by a series tuned arrangement including two cavities in the broad walls of the E-arm 1 as indicated at 5 and 6, which cavities serve as inductive elements, and two capacitance screws 7 and 8 in the junction space of the H-arm 2 through the narrow walls thereof.

Matching between the H-arm 2 and the junction is not subject to the same severe requirements as in the case of the E-arm, since it has only to deal with a single channel the band-width of which is relatively narrow as compared with that of the E-arm which has to carry the band-width covered by all the channels. It is, however, necessary that whatever matching arrangement is used for the H-arm must not aiiect the matching of the E-arm. This later condition is met if all metal components Within the junction space are both thin and are placed perpendicular to the pre-existing electric field set up by input from the E-arm, for they will then create a minimum of disturbance. Matching elements in the H-arm must, unless they satisfy the above criterion, be sufficiently far from the junction space not to affect the field therein.

In the present embodiment, it has been found that satisfactory matching of the H-arm can be obtained by the combination of an inductive element 12 in the H-arm at a suitable distance from the junction, supplemented by a pair of spaced matching screws the positions of which are indicated at 9 and 10 in Figure 4.

The matching measures described above, are primarily concerned with the separate matching of the E- and H- arms so that they give rise to no reflexions at a distance from the junction. In addition, matching post 11 (visible in Fig. 3) is provided within the junction space perpendicular to the closed end thereof. This post may be regarded as giving a certain amount of impedance matching for the H-arm of the junction, but is by itself insuflicient to cover satisfactorily the wide range of frequencies over which the junction may have to operate and it is for this reason that separate matching of the input of the E and H-arms is provided.

While the invention has been described above in connection with a specific embodiment, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What we claim is:

l. A wave guide hybrid T comprising four Wave guide sections forming the junction and comprising an input E-arm, two side arms of the junction having their longitudinal axes coplanar with that of said E-arm, and an H-arm having a closed end, all said arms being of rectangular cross-section with the ratio of the longer dimension to the shorter dimension of the rectangle being substantially equal to three to one, said E-arm extending perpendicular to and centrally from one of the broad walls of said H-arm at the closed end thereof, and both said side arms extending perpendicular to the other broad wall of said H-arm and being spaced apart substantially one third of said longer dimensions, the longer cross- 3 sectionaldimensions of said E-arm and of both said side arms all being parallel to the direction of said H-arm, said E-arm being matched in impedance by a series tuned circuit including a pair of inductive cavities located in its broad walls and means for varying the impedance 5 of'said tuned circuit.

2; An arrangement according to claim 1 in which said H-arm is provided with two capacitance screws located in the narrow walls, said screws being seriestuned with said cavities to provide impedance matching for said E-arm.

3. An arrangement according to claim 1 in which said H-arm is provided with an inductive diaphragm and is also provided with two matching screws located 4 in one of its broad walls at points outside the junction space.

4. An arrangement according to claim 1 in which said H-arm includes a post for general impedance matching.

References Cited in the file of this patent UNITED STATES PATENTS 2,643,295 Lippmann Iune 23, 1953 2,704,351 Dicke -5. Mar. 15, 1955 2,759,154 Smith Aug. 14, 1956 2,764,740 Pratt Sept. 25, 1956 2,792,551 Smith May 14, 1957 

